ext4: use s_inodes_per_block directly in __ext4_get_inode_loc

In __ext4_get_inode_loc, we calculate inodes_per_block every time by
EXT4_BLOCK_SIZE(sb) / EXT4_INODE_SIZE(sb).  AFAICS, this function is a
hot path for ext4, so we'd better use s_inodes_per_block directly
instead of calculating every time.
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -4617,7 +4617,7 @@ static int __ext4_get_inode_loc(struct inode *inode,
 	/*
 	 * Figure out the offset within the block group inode table
 	 */
-	inodes_per_block = (EXT4_BLOCK_SIZE(sb) / EXT4_INODE_SIZE(sb));
+	inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
 	inode_offset = ((inode->i_ino - 1) %
 			EXT4_INODES_PER_GROUP(sb));
 	block = ext4_inode_table(sb, gdp) + (inode_offset / inodes_per_block);